1) Field of the Invention
The present invention relates to a low-temperature curable epoxy resin curing agent comprising a polyamino compound and a curing accelerator, and an epoxy resin composition comprising said epoxy resin curing agent.
The epoxy resin curing agent is applicable to a curing agent for epoxy resin utilized in extremely wide fields including a field of coating materials such as an electrodeposition paint for motor vehicle, a corrosion-resistant paint for ships, bridges and a land and marine iron structure, and an inner surface coating paint for a beverage can; a field of materials for electricity and electronics such as a laminated sheet, a semiconductor sealant, an insulating powdered paint and a coil impregnant used in home electric appliances, communication facilities, a control system for automobiles and aircraft and the like; a field of materials for civil engineering and construction such as a material for quake-resistant reinforcement of bridges, a material for lining, reinforcement and repair of a concrete structure, a flooring material of a building, a lining of water supplying facility and sewerage, a pavior for waste water and permeating water; a field of an adhesive for vehicle and airplane, and a field of composite materials for airplane, industrial materials and sports equipment.
2) Related Art
It has widely been known that various amino compounds such as an aliphatic amino compound and an alicyclic amino compound are used as a curing agent for epoxy resin and a raw material thereof.
Among the aliphatic amino compounds, an aliphatic diamine represented by the following formula (1) has such features that, when it is used as a raw material of a curing agent, it provides a curable composition which can be cured quickly at low temperature and a cured product having an excellent chemical resistance.H2N—H2C—A—CH2—NH2   (1)wherein A is a phenylene group or a cyclohexylene group
On the other hand, however, the diamine represented by the formula (1) easily produces carbamate by absorbing carbon dioxide or water vapor in the atmosphere which causes such defects that the phenomena of whitening or stickiness tends to appear on an epoxy resin cured coating film wherein the diamine is used as a curing agent.
Consequently, as a curing agent for epoxy resin, the diamine represented by the formula (1) is scarcely used as it is without modification. In most cases, they are used after various modifications.
Representative methods for modification of diamines include a modification by reaction with a compound having a carboxyl group, a modification by reaction with an epoxy compound, a modification by Mannich reaction with an aldehyde compound and a phenolic compound, a modification by Michael reaction with an acryl compound and the like.
Among these various methods for modification, a reaction product of Mannich reaction of an amino compound with an aldehyde compound and a phenolic compound has a feature that its curing speed is high. Especially, a reaction product of Mannich reaction of diamine represented by the formula (1) is widely utilized as an epoxy resin curing agent in the field wherein a low-temperature curability is required. (See “New Development Of A Curing Agent For Epoxy Resin” edited by Hiroshi Kakiuchi, published by CMC Co. Ltd., P88, May 31, 1994).
In general, it is preferable that the viscosity of a curing agent is relatively low from the viewpoint of workability. Since a reaction product of Mannich reaction is a modified product of an amino compound with an aldehyde compound and a phenolic compound, it is possible to obtain a Mannich reaction product having a low viscosity by reducing the reaction mole ratio of the aldehyde compound to the amino compound or by increasing the reaction mole ratio of the phenolic compound to the amino compound. However, a relatively large amount of unreacted phenolic compound remains in a low-viscosity Mannich reaction product obtained by such modification.
At present, phenol is generally used as a raw-material phenolic compound of the Mannich reaction product. However, since phenol is designated as a poisonous substance and besides, its mutagenicity is identified, it has come to believe that unreacted phenol should not remain in the Mannich reaction product. Thus, a strong tendency not to use phenol has been developed recently.
In order to reduce the amount of unreacted phenol remained in the Mannich reaction product, a method such as increasing the reaction mole ratio of an aldehyde compound to an amino compound or decreasing the reaction mole ratio of a phenolic compound to an amino compound can be applied. However, the viscosity of a Mannich reaction product obtained by such methods becomes high. When the high-viscosity Mannich reaction product is used as a curing agent or a raw material thereof, it is necessary to dilute the product by adding a solvent in order to improve workability.
Recently, there is a strong tendency not to use solvents in an epoxy resin curing agent for a coating material in order to prevent the global environmental pollution, and it is desired to develop an epoxy resin curing agent which is available in a non-solvent condition.
An amino compound obtained by modifying metaxylylenediamine and the like is referred as an epoxy resin curing agent in Japanese Patent Kokai (Laid-open) No. 2002-161076, p3-5, and it discloses that said amino compound has relatively low viscosity.
However, it is described in the above document that said amino compound, when it is used as an epoxy resin curing agent, has a long pot life at room temperature. Therefore, it is believed that, when said amino compound is used as a curing agent, curing reaction would not progress at low temperature.